He gave you a link to the way to do it. You are comparing a string to an integer, and that is why you are getting an error. First you must convert the string to an integer. Try googling it and you won't have a hard time figuring it out.

if ( number < 0)
i+=1; // only line of code in your if statement
{ // completey separate block of code starts here
System.out.println("You must enter an integer between 0-100");
SimpleIO.prompt("Enter a number 0-100");
} //ends here